This invention relates to color cathode ray picture tubes of the type having a shadow mask for color selection, and is particularly concerned with means and method for establishing the mask and imaging screen at the proper electrical potential.
The area enclosed by the funnel and face panel of a color cathode ray picture tube is typically established as a field-free region for the excursion of the electron beams that selectively excite the pattern of discrete phosphor targets deposited on the inner surface of the face panel that comprises the picture imaging screen. The field-free region is established by charging the inner surfaces of the funnel and face panel and adjacent components to a high potential, typically in the range of 25-32 kilovolts. The surfaces so charged include the conductive coating deposited on the inner surface of the funnel and an electrically conductive film, normally aluminum, disposed on the back of the picture imaging screen. The shadow mask is also charged to the same high potential. The shadow mask, which may have a frame, or be of the frameless type, is typically suspended adjacent to the face panel by a plurality of spring means attached at one end to a shadow mask supportive member such as the frame, and at the opposite end to an electrically conductive pin extending from the face panel flange, and in electrical contact with the imaging screen and its electrically conductive film. The electrically conductive coating on the inner surface of the funnel receives the high potential from a metallic "anode button" that protrudes through the wall of the funnel, and which in turn is connected to a conductor leading from a power supply.
The electrical path between the electrically charged funnel coating and the shadow mask and screen is typically supplied by means of an electrical bridge comprising a flexible spring, one end of which is commonly welded to the mask frame, and with a distal end spring-biased and suitably shaped to make contact with the electrically charged conductive coating on the funnel. This bridge is commonly termed an "anode contact spring."
The process by which the anode contact spring is attached to the shadow mask is usually by locating the spring by eye and fastening it by spot welding. This process has presented problems. For example, proper placement of the spring is operator-dependent, and an error in location can result in an installation in which the anode contact spring may make little or no contact with the funnel conductive coating. Also, "weld splash" consequent to the welding process can occlude one or more of the apertures of the nearby foraminated section of the mask. The occlusion of only one such aperture is clearly visible to the viewer. Further, if it is necessary to salvage the shadow mask, removal of the welded-on bridging means is difficult and its dislodgment can result in distortion and destruction of the mask.
Installation of the standard bridging spring on a shadow mask of the frameless type has also presented a problem. A mask of this type is disclosed in U.S. Pat. No. 4,100,451 to Palac, of common ownership herewith. The frameless mask comprises a low-cost, light weight, non-self-rigid, torsionally flexible mask preferably of one-piece construction. Because of the absence of a frame and the thinness of the mask material (of the order of six mils), a bridging means cannot be attached to the mask by conventional means. Attachment to the very thin metal of such a mask by welding for example would most likely result in a very light, inadequate pressure against the electrically charged inner surface because of mask flexing. Also, the mask itself could be deformed, and welding to the very thin metal of the mask is considered impractical.
In the application Ser. No. 584,566 filed June 4, 1975, now abandoned, of common ownership herewith described in British Patent Specification No. 1,503,865, an electrical bridge comprising an anode contact spring is disclosed for use with a color television picture tube having a flangeless faceplate and a shadow mask of the aforedescribed frameless type. At least one electrically conductive shadow mask support stud projects from the faceplate inner surface. The contact spring comprises a sleeve-like base for making slidable frictional engagement with the stud, and a spring member extends from the base for resiliently contacting the electrically charged funnel coating. The sleeve-like base may have a rectangular cross-sectional configuration substantially matching the cross-sectional configuration of the stud, but slightly larger to provide for outside telescoping engagement of the base with the stud. The base may be welded to the stud after such engagement.
Japanese Patent No. 48-12959/73 (JPC 99 F 120.2) discloses an electrically conductive contact spring affixed to an electrically conductive element. As best understood by examination of the drawing, it appears that the contact spring extends from a shadow-mask-supporting stud.
U.S. Pat. No. 3,541,373--Barr discloses an electrical bridge comprising a contact spring which is affixed to the frame of a shadow mask. The spring is shown as being formed with a clip portion which appears to snap over the edge of a flange of the frame and an associated electron beam shield. Although not shown, it is assumed that the attachment means is made more secure by an in-turning flange at the end of the spring to engage the turned-in rim of the shield. The security of the attachment is believed to be defeated by the fact that the force exerted by the distal end of the bridge against the funnel in making contact with the conductive coating is in the direction of the force required to unclip the spring. Additionally, the bridge would seem to be vulnerable to accidental dislodgement. It is believed that for this reason, an alternate and more secure method of attaching by welding is taught.